CO2 conversion to VFAs via microbial electrosynthesis

Microbial electrosynthesis (MES) is a potential technology for CO2 recycling into valuable chemical products, but its development is hindered by an insufficient information on the microbial metabolism and interactions underpinning the electrochemically-assisted reactions. In this study, a MES reactor was operated for a total of 225 days with either bicarbonate or CO2 as carbon source, under batch and continuous feeding regime. A stable acetic acid production with a rate of 9.68 g/m2/d, with coulombic efficiency up to 40%, was achieved with continuous CO2 sparging, substantially higher than the rates obtained with bicarbonate (0.94 g/m2/d) and CO2 (2.54 g/m2/d) in fed-batch regime. However, the highest butyric acid production rate of 0.39 g/m2/d was achieved with intermittent CO2 sparging. A microbial community analysis was conducted focusing on amplicon sequence variants (ASVs) significantly different between samples collected under different feeding regimes. Such analysis revealed that acetogenesis was carried out by Clostridium sp., Eubacterium sp. and Acetobacterium sp., whereas Ruminococcaceae such as Oscillibacter sp. and Caproiciproducens sp. were involved in butyric acid produciton. Rikenellaceae, found strongly upregulated in the cathodic than in the planktonic community, were most likely the main contributor to H2 production at the cathode. The abundance of Acetobacterium sp. positively correlated with the microaerobic microorganism Sulfurospirillum sp., highlighting the syntrophic metabolism of the two organisms.